It is common for communications systems to have a digital part in which a signal to be transmitted is processed before transmission and a further digital part in which received signals are processed after reception. Processing in the digital parts is typically carried out at base band, that is to say at the frequency band of signals before any offset for the purpose of transmission at a carrier frequency; generally base band signals encompass zero frequency components, i.e. direct current (DC) components. It is common for base band signals to be represented by in-phase (I) and quadrature (Q) parts, that is to say, a complex representation. The processing may comprise such procedures as filtering, modulation demodulation coding and decoding. It is generally necessary to convert signals to and from the analogue domain for transmission and reception, and in the case of wireless systems, it is necessary to convert signals to and from an appropriate radio frequency.
There are various approaches to conversion of digital signals from base band to and from a radio frequency. One approach is to up convert in the digital domain, so that complex base band signals are multiplied, that is to say mixed, by a digital local oscillator to produce an output at a higher frequency, often called an intermediate frequency (IF), that may then be converted to the analogue domain by a digital to analogue converter. The intermediate frequency signal is a real-only, rather than complex, signal. The analogue signal may then be further frequency translated to an appropriate frequency for transmission. Similarly on reception, signals are converted from the analogue to digital domains at an intermediate frequency, higher than base band, and then mixed down to in-phase and quadrature base band signals digitally. An advantage of this approach is that the conversion from base band complex signals to intermediate frequency signals and vice versa is carried out digitally and so is not subject to analogue errors that may cause differences in response between in-phase and quadrature channels. However, a disadvantage is that digital to analogue converters and analogue to digital converters have to operate at a higher frequency than base band, in order to convert intermediate frequency signals. Operating these components at a higher frequency means that the components are costly, and potentially of lower performance in terms of resolution than lower frequency digital to analogue converters and analogue to digital converters.
An alternative approach to the conversion of digital signals from base band to and from a radio frequency is generally termed direct conversion. In a direct conversion architecture, the base band in-phase and quadrature signals are converted to and from analogue form at base band. On transmit, the analogue in-phase and quadrature signals are then up-converted in the analogue domain by analogue quadrature mixers. Preferably, the up-conversion is to the radio frequency transmission frequency in one step, and as a result use of an intermediate frequency is not required. Similarly, on reception, conversion of received radio frequency signals is preferably directly to in-phase and quadrature base band analogue signals, that are then converted to the digital domain. An advantage of the direct conversion approach is that appropriate digital to analogue and analogue to digital converters may be less costly, and of higher performance in terms of resolution. Also, the omission of the intermediate frequency stage can lead to cost savings due to the need for fewer components. However, there is a potential penalty in that the in-phase and quadrature signal paths involve analogue components such as filters that are subject to variation of component values within a tolerance, so that the analogue properties of the in-phase and quadrature signal paths may vary from equipment to equipment and over temperature.
Errors that cause degradation from the perfectly orthogonal in-phase and quadrature channels that exist in the digital domain are known as quadrature errors, or IQ errors. In particular, there may be problems if there are differential errors between in-phase and quadrature channels. Differential errors between in-phase and quadrature channels may cause, for example, spurious components to be generated in a transmitter and spurious responses in a receiver. In particular, a spurious response may be generated in the opposite side band to that intended; for example, if a signal component is intended to be at a higher frequency than a local oscillator signal, then a differential error between in-phase and quadrature components may lead to a spurious component appearing at a lower frequency than that of the local oscillator signal.
Quadrature errors typically comprise voltage offsets, that is to say DC offsets, differential gain characteristics between in-phase and quadrature signal paths, and phase error between in-phase and quadrature signal paths. A conventional quadrature correction network for the correction of such quadrature errors is illustrated in FIG. 1; gain correction blocks Igain and Qgain are shown, as is a block for the correction of phase errors between in-phase and quadrature paths, marked IQ phase, and blocks for the correction of DC offsets I DC Offset and Q DC Offset.
However, quadrature errors, and in particular differential quadrature errors, in both the upconversion and downconversion may be dependent on frequency within the base band. For example, analogue filtering may introduce such errors, particularly in anti-aliasing filters, due to the variation of the values of analogue components within component tolerance limits and with temperature. Conventional correction networks cannot correct such errors.
The present invention addresses these disadvantages.